SUMMARY Chemical Targeting of Sensors and Pharmacological Probes in the Brain There are no general tools that enable monitoring and/or modulation of specific circuits and cells in the mammalian brain by purely chemical means. We have recently disclosed a polymer platform for chemical delivery of voltage sensitive dyes (VSDs) to specific cell types in the brain (monoaminergic neurons and their extensions), as a first example of a non-genetic system enabling targeted delivery of highly lipophilic molecules in brain tissue. We have shown that dextran, a bacterial polysaccharide, can function as a polar polymer carrier which dynamically encapsulates the lipophilic VSD to carry it through brain tissue and deploy it at a specific cell type of interest by the means of a homing ligand. The proposed research aims to build on this proof-of-concept example and develop a general platform for delivery of lipophilic sensors and actuators/drugs to specific cell types in the brain. In Aim 1A we will develop traceless covalent labeling techniques based on ligand-directed acyl imidazole chemistry to a) enable cell-type specific targeting without long-term perturbation of the system by pharmacological effects of the ligand, and b) covalently immobilize voltage sensors in the vicinity of ion transporting proteins (AMPA receptor and dopamine transporter) to measure local changes of membrane potential. In Aim 1B, we will leverage the pharmacological effects of the ligands by extending the platform to dual-drug targeting which will enable delivery of dopamine receptor D2 agonists specifically to D2-expressing medium spiny neurons (D2-MSNs), but not dopaminergic axons in the striatum, by the means of an adenosine receptor A2A antagonist (a D2-MSN marker). This will provide a platform for cell selective pharmacology with therapeutic potential for neurodegenerative (e.g. Parkinson?s disease) and psychiatric disorders. In Aim 2 we focus on a) optimization of fluorescent VSDs for high sensitivity and targetability using the dextran platform; and b) extension of the targeting platform to voltage sensors enabling novel imaging modalities including short wave infrared fluorescence microscopy and photoacoustic imaging. In Aim 3 we will systematically optimize the dextran delivery platform by tuning the polymer molecular weight and substitution pattern. We will also explore other saccharide-based polymers as delivery systems for lipophilic cargo in the brain, including cyclodextrins and poly(styrene-ether-trehalose). The outcome of this 5-year program promises to provide a general and optimized platform for delivery of lipophilic sensor and actuators to specific cell types in the brain. The chemical targeting approach eliminates the need for genetic manipulation of the brain and thus will be, in a long-term perspective, applicable to humans and other organisms not readily adaptable for genetic manipulations.